47 C.F.R. § 1.717
The Commission will forward informal complaints to the appropriate carrier for investigation and may set a due date for the carrier to provide a written response to the informal complaint to the Commission, with a copy to the complainant. The response will advise the Commission of the carrier's satisfaction of the complaint or of its refusal or inability to do so. Where there are clear indications from the carrier's response or from other communications with the parties that the complaint has been satisfied, the Commission may, in its discretion, consider a complaint proceeding to be closed. In all other cases, the Commission will notify the complainant that if the complainant is not satisfied by the carrier's response, or if the carrier has failed to submit a response by the due date, the complainant may file a formal complaint in accordance with § 1.721 .
47 C.F.R. §1.717